Vibrating beam force transducers are often used as force-to-frequency converters in accelerometers, pressure sensors and related instruments. In one well-known design, described in U.S. Pat. No. 4,372,173, the force transducer is in the form of a double-ended tuning fork fabricated from crystalline quartz. The transducer comprises a pair of side-by-side beams that are connected to common mounting structures at their ends. Electrodes are deposited on the beams in predetermined patterns, and the electrodes are connected to a drive circuit. The drive circuit provides a periodic voltage that causes the beams to vibrate toward and away from one another, 180.degree. out of phase. In effect, the drive circuit and beams form an oscillator, with the beams playing the role of the frequency control crystal, i.e., a mechanical resonance of the beams controls the oscillation frequency. A tension force applied along the beams increase the resonant oscillation frequency. The frequency of the drive signal is thereby a measure of the force applied axially along the beams.
Vibrating beam force transducers require materials with low internal damping, to achieve high Q values that result in low drive power, low self-heating, and insensitivity to electronic component variations. Transducer materials for high-accuracy instruments also require extreme mechanical stability over extended cycles at high stress levels. One of the key problems in producing such transducers involves the drive and position pick-off measurement. Crystalline quartz is the most commonly used material for mechanical transducers because of its piezoelectric properties, which properties provide the ability to drive and sense mechanical motion through the use of a simple surface electrode pattern.
With the advent of low cost, micromachined mechanical structures fabricated from crystalline silicon, it has become desirable to create silicon vibrating beam transducers. However, silicon does not possess piezoelectric properties for driving and sensing beam vibration. It was therefore desirable to provide a method of exciting and sensing the resonance of a silicon beam, without adding substantial costs, mechanical instabilities, or excessive complexity. One prior approach to this problem was described in U.S. Pat. No. 4,912,990, issued to the assignee of the present invention. The invention described in the '990 patent provides a vibrating beam force transducer that can be realized in a silicon micromachined structure. The force transducer is of the type comprising a beam having a longitudinal axis, and drive circuitry electrically coupled to the beam for causing the beam to oscillate at a resonant frequency that is a function of a force applied along the longitudinal axis of the beam. Magnetic means are provided for creating a magnetic field that intersects said axial component. Motion of the beams in the magnetic field generates a signal voltage. This voltage is amplified, and the amplified voltage drives current along a conduction means physically coupled to the beam. The electric current flowing along the current path thereby interacts with the magnetic field, so as to produce a force on the beam that causes the beam to oscillate at the resonant frequency.
One difficulty which has arisen with respect to the drive means utilized in the prior art concerns variations caused by temperature cycling of the transducer and its drive circuitry.
In a particular drive circuit utilized in conjunction with a doubled-ended tuning fork (DETF) in a force transducer of the type described in the '990 patent, the tuning fork is covered by oxide and a conducting layer of gold is applied over the oxide to provide an electrically conducing path. This conducting path traverses from a first end of one tine to the other end of the tuning fork, across the adjacent end of the other tine, and back along the second tine to the end of the second tine adjacent to the first end of the first tine. There is a voltage difference between electrical connections to the ends of this conducting path which comprises two components: a first component generated by the motion of the tines in the magnetic field; and a second component caused by the flow of the drive current through the electrical resistance of the conducting path and any leads between the connections and the tines. A bridge is formed of two voltage dividers, one of which includes the conducting path along the tines and any leads, permits a differential amplifier to subtract out the second voltage component. The resistance of the bridge components can change with temperature. The problem arises most particularly with respect to the resistor in series with the double-ended tuning fork. The gold conducting path on the tuning fork, and any gold leads from the resistor in series with this path, change resistance with temperature, so that the voltages from the two voltage dividers are not the same, in which case the effective electrical Q of the resonator is significantly degraded, and the oscillator works poorly or does not work at all. One method which can be utilized to compensate for this is by forming the appropriate resistor of the parallel divider by depositing gold on an insulating outside layer of the silicon at the same time the gold conducting path on the tuning fork is deposited. It can be difficult or impractical to apply the gold so that its change in resistance matches that of the gold on the tuning fork. Also, this may then necessitate trimming of the resistor, which is an undesireable requirement in production.
It therefore is desirable to provide an electric drive circuit for the magnetically driven force transducer which compensates for any differences in resistance which may occur as a result of temperatures cycling over the operating range of the device.